1. Field of the Invention
The present invention relates to a method for evaluating changes in the resistance of an electric resistance member, and more particularly to a method for evaluating the electric charge carrying member of an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as copiers, facsimile devices, and printers are provided with an image carrier, a latent image carrier, and a developing agent carrier as members that are electrified and carry an electric charge. Specific examples of such carriers include a photosensitive body, a developing roller, and a transfer member. In case of image forming apparatuses comprising multiple toners, such as color imaging apparatuses, a transfer belt is used as the transfer member.
Tandem color image forming apparatuses are known as the image forming apparatuses having a transfer belt. The tandem color image forming apparatuses are provided with a plurality of photosensitive bodies disposed along the surface of the transfer belt as image carriers. Such a transfer belt is provided with an electric charge with transfer bias application means and carries the charge. A method of using toner images of multiple colors which are formed on the photosensitive bodies to obtain a color image directly on the transfer material which is supported and transported by the transfer belt and a method of indirectly transferring a toner image on a transfer belt as an intermediate transfer belt and then obtaining a color image by transferring the image to the transfer material are known.
Constant voltage control means which maintains a constant voltage applied to the image carrier and constant current control means which maintains a constant current flowing in the image carrier are employed as transfer bias application means for providing the transfer belt with an electric charge. Values of the voltage and current which are controlled vary depending on the surface resistivity or volume resistivity of the belt. For this reason, the resistance value of the belt is required to be uniform in the circumferential direction and the dependence thereof on environment and voltage has to be low.
However, the following problems were associated with the transfer belts. Thus, when the transfer belt was continuously provided with an electric charge for a long time, the charge remained on the boundary surface of the transfer belt and the resistance value of the transfer belt increased. Furthermore, the transfer belt was degraded, for example, by an electric discharge between an image carrier and a transfer roller, which resulted in the formation of a conductive path on the transfer belt and decreased the resistance value of the transfer belt. Thus, because the resistance value of the transfer belt changes with time, the resistivity thereof will change if such a belt is used for a long time. As a result, the quantity of electric charge supplied to the transfer belt could be insufficient and the image density could decrease due to such an insufficient transfer or, conversely, the quantity of electric charge supplied to the transfer belt could be too high, causing partial discharge and transfer loss which corresponds to the discharge.
Furthermore, when changes in the resistance value of such a transfer belt with time were measured, a test comprising a step of applying a voltage within a fixed interval to the belt was repeated hundreds of times to measure the changes in the resistance value of the transfer belt with time. For this reason, a long time was required to determine the changes in the resistance value of one belt with time.
A differential constant current control method that was conducted so as to obtain a constant difference between the output current from a transfer bias source and a current returning via the transfer belt and a method for preventing image degradation by a constant voltage control based on the correction of current value, as described, for example, in Japanese Patent Applications Laid-open Nos. 2001-209233, 2000-147849, 2001-125338, and H8-194389, have been employed when the volume resistivity of the transfer belt has changed. However, when the surface resistivity of the transfer belt changes with time, the leak quantity of electric current in the circumferential direction of. the belt varies. As a result, a problem associated with power sources for supplying a bias to bias application means in each transfer location rises, for example, in case of tandem color image forming apparatuses with four transfer locations. The problem is that even if the leaking current is detected from transfer locations of the transfer belt, it is unclear from which location the current has leaked. As a result, it was not clear which power source output has to be controlled and how it should be done, and the differential constant current control or constant voltage control based on current value correction could not be used.
Problems associated with the transfer belt as a transfer member were described above, but problems caused by changes in the resistance value with time are also associated with electric resistance members. In particular, the problems are especially easily caused by changes in the resistance value of a photosensitive body, which is a change carrying member of the image forming apparatus, or a development roller.